This writer's after office material (5)
I believe the answer is:
denim
'material' is the definition.
(denim is a kind of material)
'this writer's after office' is the wordplay.
'this writer's' becomes 'im' (the person writing the crossword clues**).
'after' is a charade indicator (letters next to each other).
'office' becomes 'den' (I've seen this before).
'im' put after 'den' is 'DENIM'.
(Other definitions for denim that I've seen before include "Hard-wearing blue fabric" , "Hard-wearing material seen in jeans" , "Twill cloth" , "Material for casual trousers or jackets" , "Mined (anag)" .)